The potential difference between the inside & outside of the earth's crust is different. Charge resides on the outside of a sphere. The sphere's integrity is locally broken in a volcanic eruption enabling lightning discharges from the troposphere via very hot & ion rich lava & vog all the way down to real earth potential.

Electrical events are everywhere in the universe. Astronomers should be aware of that.

The potential difference between the inside & outside of the earth's crust is different. Charge resides on the outside of a sphere. The sphere's integrity is locally broken in a volcanic eruption enabling lightning discharges from the troposphere via very hot & ion rich lava & vog all the way down to real earth potential.

This lightning has nothing to do with potential between the inside and outside of the Earth. It's created substantially the same way all lightning is created- by charge separation mechanisms inside clouds. In this case, it's a dust cloud rather than a water cloud.

The potential difference between the inside & outside of the earth's crust is different. Charge resides on the outside of a sphere. The sphere's integrity is locally broken in a volcanic eruption enabling lightning discharges from the troposphere via very hot & ion rich lava & vog all the way down to real earth potential.

This lightning has nothing to do with potential between the inside and outside of the Earth. It's created substantially the same way all lightning is created- by charge separation mechanisms inside clouds. In this case, it's a dust cloud rather than a water cloud.

What about within the coma of a comet or a volcanic eruption on an atmosphere-less (or near to) body such as the earth’s moon?

The potential difference between the inside & outside of the earth's crust is different. Charge resides on the outside of a sphere. The sphere's integrity is locally broken in a volcanic eruption enabling lightning discharges from the troposphere via very hot & ion rich lava & vog all the way down to real earth potential.

This lightning has nothing to do with potential between the inside and outside of the Earth. It's created substantially the same way all lightning is created- by charge separation mechanisms inside clouds. In this case, it's a dust cloud rather than a water cloud.

What about within the coma of a comet or a volcanic eruption on an atmosphere-less (or near to) body such as the earth’s moon?

Even if you have a charge separation mechanism (not obvious what that would be in a comet's coma, although it's possible in any volcanic eruption that produces thick dust), you still need a medium dense enough to ionize and provide a discharge path. That's not going to be present around a comet or on an airless body.

Why is volcanic lightning still a mystery in 2018 A.D.? Am I missing something here? The source / cause is pretty much the same as storm lightning - turbulent motion of a cloud of polar particles, namely water droplets, plus, in the case of a volcano, silicate dust particles just pulverized.

The most abundant gas in a volcanic eruption, somewhere in excess of 90% of the entire gas budget in most eruptions (there are a few rare exceptions), is water. That percentage exceeds 99% in basaltic eruptions, especially on ocean islands. In fact, the only reason any magma, especially a low viscosity one such as basalt, produces ANY cloud at all is due to the presence of water either in the magma or coming in contact with the magma in the substrate.

Volcanology lesson aside, volcanic clouds obviously are more (rock) dust laden than are atmospheric clouds, so the presence of micron to submicron dustparticles increases overall electrical charge density, meaning that lightning is more likely in a volcanic cloud than in an atmospheric one. Eruptions are fairly rare and someone witnessing or recording lightning is rare, and rare times rare equals rare-squared, but volcanic lightning isn't rare when the sample set is limited to volcanic eruptions, especially those that involve enough water to produce a cloud.

Why is volcanic lightning still a mystery in 2018 A.D.? Am I missing something here? The source / cause is pretty much the same as storm lightning - turbulent motion of a cloud of polar particles, namely water droplets, plus, in the case of a volcano, silicate dust particles just pulverized.

I would distinguish between either being a "mystery" (which isn't the case) and each having many elements which remain poorly understood (which is quite true).